1. Field of the Invention
The present invention relates in general to cathodic protection systems and in particular to an improved electronic control circuit for such a cathodic protection system.
Corrosion is a well known and continuous process which results in the destruction of metals. In order for metal to corrode, there must be a corrosion cell, consisting of a metallic anode, a metallic cathode, and a chemical electrolyte between the anode and the cathode. There must also be an electric potential difference between the anode and the cathode. When this condition occurs, positively charged atoms of the metal leave the surface of the anode and enter the electrolyte as metallic ions. The ions migrate through the electrolyte to the cathode. As a result, corrosion occurs at the anode. Consequently, it will be appreciated that corrosion occurs as a direct result of an electric current passing between two metallic surfaces.
As is well known, vehicles, such as automobiles and boats, are particularly subject to corrosion. This is true because dissimilar metals are generally utilized in the construction thereof. The dissimilar metals inherently create a difference in electric potential therebetween. Also, various chemicals in the air or in water provide the chemical electrolyte between the dissimilar metals. Consequently, portions of the metallic body of the vehicle function as anodes and, therefore, are destroyed by the above described corrosion process.
Cathodic protection systems are well known in the art for retarding the process of corrosion on vehicles. Such systems generally provide one or more sacrificial anodes which are secured to the vehicle. A relatively small electric current is passed through the sacrificial anode to the vehicle. The flow of the electric current raises the relative electric potential of the sacrificial anode above that of the vehicle. Hence, the vehicle acts as the cathode of the corrosion cell described above and, therefore, is less subject to corrosion than would otherwise occur.
2. Description of the Prior Art
U.S. Pat. No. 4,528,460 to Staerzl discloses a control system for cathodically protecting an outboard drive unit from corrosion. The control system includes an anode and a reference electrode mounted on the drive unit. Current supplied to the anode is controlled by a transistor, which in turn is controlled by an amplifier. The amplifier is biased to maintain a relatively constant potential on the drive unit when operated in either fresh or salt water.
U.S. Pat. No. 4,510,030 to Miyashita et al. discloses a method for the cathodic protection of an aluminum article. The cathodic potential of the article is observed relative to a reference electrode. When the observed cathodic potential approaches the level at which corrosion occurs, the article is electrically connected to a source of electrical voltage to repress the cathodic potential of the article. After a sufficient repression, the article is disconnected from the source, whereby the cathodic potential of the article gradually rises again. Thus, the article is intermittently connected to the source of electric potential.
U.S. Pat. No. 4,322,633 to Staerzl discloses a marine cathodic protection system wherein a reference electrode senses the electric potential near a submerged portion of a drive unit. The signal from the reference electrode is supplied to an operational amplifier, where it is compared with a reference voltage. If the potential sensed by the reference electrode is greater than a predetermined magnitude, the operational amplifier generates an output signal which prevents the flow of electric current to an anode.
U.S. Pat. No. 4,647,353 to McCready discloses an anode structure for an automotive cathodic protection device. It is stated that the device incorporates a pulsating system that allows a vehicle battery (which is utilized as a source of energy for the cathodic protection device) to use its rejuvenating properties more effectively.
U.S. Pat. No. 4,311,576 to Toudo discloses an electric corrosion preventing apparatus including a lamp which is energized when the apparatus is in use.
Other related cathodic protection systems are disclosed in U.S. Pat. Nos. 3,830,719 to Cavil, 4,351,703 to Winslow, Jr., 4,409,080 to Slough, and 4,492,877 to Staerzl.